PREPARATION OF LEAD IODIDE AS INPUT MATERIAL FOR X-RAY DETECTORS

1 1 1 1 2M. MATUCHOVÁ , O. PROCHÁZKOVÁ , K ŽÏÁNSKÝ , J. ZAVADIL , J . MAIXNER , 1Institute of Radio Eneineeruing and Electronics. Czech Academy of Science, Chaberská 57, 182 51 Praha 8

2Institute of Chemical Technology, Technická 5, 160 28 Prague, Czech RepublikE-mail:Marie.Matuchova@tiscali.cz

PREPARATION OF LEAD IODIDE AS INPUT MATERIAL FOR X-RAY DETECTORS

1 1 1 1 2M. MATUCHOVÁ , O. PROCHÁZKOVÁ , K ŽÏÁNSKÝ , J. ZAVADIL , J . MAIXNER , 1Institute of Radio Eneineeruing and Electronics. Czech Academy of Science, Chaberská 57, 182 51 Praha 8

2Institute of Chemical Technology, Technická 5, 160 28 Prague, Czech RepublikE-mail:Marie.Matuchova@tiscali.cz

(100)

(-111) B

(-11-1)

(1-11) B

(1-1-1)

(100)

(-111) B (1-11) B

(1-1-1)(-11-1)

Lead iodide (PbI ) belongs to one of promising materials for high efficiency uncool solid state detectors (in the range of 1kev-1MeV) operating at room temperatures. It can be 2

applied over a wide temperature range from 200°C up to 130°C in detectors in devices used within and outside the laboratory, for example, for ecological measurements (poluted waste water, sewage, etc.), and for improved diagnostic methods in biology and medicine (radiography and tomography). Lead iodide is often compared with mercuric iodide. Especially two important physical properties make PbI a more interesting material for detector applications than HgI These are its lower vapour pressure, and its higher chemical 2 2 .

stability. No degradation was observed in the PbI detectors under laboratory ambient in 6 month. The polytypism of PbI seems to be also one significant property of this material. 2 . 2

PbI has not a structure modification change.2

The chemical analysis and stoichiometry has been determined by X-Ray spectrometer ARL 9400XP. The specific resistance and photoluminescence measurement has been also measured.

The aim of this work is to study the new method of direct synthesis of lead and iodine as the input material of PbI . This method has not been studied for this material till now, and 2

seems to be one of the new methods for preparation of the input material. The photoluminescence measurement and measurement of specific resistivity has been done and compared with the measurements done by precipitation and zone purification.

1. Introduction

2. Aims

This work is supported by the Grant Agency of the Czech Republic, Projects No.102/01/1338, No.102/03/0379 and by Academy of Sciences of the Czech Republic, Project KSK 1010104

Fig. 2 Apparatus for direct synthesis (back part) and zone melting

3. MethodsBasic data of PbI2

Crystallographic system hexagonal type of CdI ( Fig.1) 2

Space group P3m1Space parameters a = 0.4557 nm

c = 0.6979 nmPbI forms polytypes: the crystals can have different c parameter 2

32 polytypes known, basic polytype 2H

Characterization of PbI2

Molecular Weight 461.00894Melting point of Pb 327 C Melting point of PbI 408 C2

Boiling point of PbI 954 C2

G of PbI starts to be negative at 500 C,2

the chemical reaction is possible from this temperature

Possible sources of PbI material 2

· commercial material of PbI the commercial material can not be used for the disturbed stoichiometry2:

· material prepared by precipitation of Pb(NO ) and KI : Using the precipitation method the stoichiometry can be 3 2

experimentally adjusted to the theoretical. The precipitated material must be purified by zone melting further.· direct synthesis of PbI using elements Pb and I Direct synthesis is experimentally more demanding than the 2 2

preparation of the input material by precipitation. However, it is possible to restrict or even dispense with the zone-melting purification step (Fig. 2).

Method of direct synthesisThe method of direct synthesis is based on the direct synthesis of the elements Pb and I by the appropriate 2

technological parameters. The reaction is feasible and sufficiently fast. The temperature is kept by the temperature controller The apparatus consists from the following main parts: regulator, quartz glass resistance furnace

A special constructed ampoule for synthesis with technological parameters in both sections is in Fig. 3. The -6 ampoules are evacuated to 10 mbar.

Fig.1 PbI crystal structure2

Fig.3 Direct synthesis of PbI , quartz glass ampule2

ExperimentsThe small inside ampoule with iodine is broken before starting

experiment. The glass ampoule is heated to 200 C in iodine section and to 700 C in lead section. PbI is formed in lead 2

section. The duration of synthesis is approximately one week. The temperature is controlled by EUROTHERM controller. The synthesized material is demonstrated in Fig. 4. Comparison with the material after precipitation and zone melting. is in Fig.5.

Fig. 4 PbI material after synthesis2 Fig. 5 PbI prepared by precipitation and 2

purification by zone melting

4. Results and discussions

Electrical measurementsMaestro software for Windows has been used. The electrical characteristic current versus voltage has been measured by the room temperature using the unit source-meter of the Keithly firm “236 source Measure Unit” in the range from 0 to 100 V with the positive and negative polarity as is shown in the Fig. 6. The resulted resistivity is estimated from the linear parts of the characteristics, which are in the region of the higher voltage. The value of the specific resistivity by both polarities is the same 1.0 x 10/ 10 ohm cm. Two measurements are compared: the sample after direct synthesis (Fig. 8.1.) with the sample (Fig. 6.2.) prepared by precipitation and purified by zone melting (10 passes and the movement of the zone 30mm/h). The resistivity for the sample from one melting is about 2 times higher. It means that purity is higher than by direct synthesis. However, the differences are not so essential and direct synthesis is comparable with precipitation. It is supposed even to purify the samples after direct synthesis by zone melting. Otherwise the number of passes though the zone can be sufficiently lowered. The arrangement of the contact and the plate is in Fig..9

Microscopic measurementsThe microscopic measurements present the layered structure of material (Fig.6a, 6b, 7) Equipment: microscope VEGA TS 5136MM, LVSTD (Low Vacuum Secondary Tescan Detector) has been used for microscopic measurements. It enables the detection of secondary electrons by the condition of low vacuum in the microscope chamber and the detector BSE for back reflected electrons for the contrast observation. Till now we could not go to atomic structure because of the lack of appropriate detector. The analytical analysis has been done as well, but not with the success. The recelebration of the analyser must be done. Several pictures are presented. The height of steps has been calculated. Fig. 6a Layered

structure of PbI2

Fig. 6b Layered structure of PbI2

Fig. 7 Steps and kinks on the structure

Low temperature photoluminiscence measurementsLow temperature PL spectra of typical PbI crystals prepared by precipitation followed 2

by zone melting (8.1.) and by direct synthesis (8.2.) are shown in Figs. 8. Application of zonal melting leads to the narrowing of spectral bands and appearance of fine spectral features, characteristic of pure material, low in defects. Categories: band-edge (BE) transitions at about 2.49 eV and free to shallow donor levels (F-B) related transitions at 2.46, 2.43 or 2.38 eV. The high energy band (BE) in Fig. 8 exhibits superlinear behaviour with increasing excitation power and results from the decay of excitons. Phonon assisted transitions associated with exciton recombination as well as with shallow defect level (2), and involving 5-8 meV LO phonons, could also be seen in Fig. 10.

Fig. 8.1 Specific resistance estimationElectrical characteristics current-voltage

Fig. 9 Arrangement of the contact and the plate

5. Discussion

CONCLUSIONThe method of the direct synthesis of lead iodide has been successfully implemented. The technology has been worked out, the analysis, and the characterization has been introduced to measure the quality of this material.

The method of direct synthesis has been compared with the method preparing material by precipitation and subsequent purification. The first measurements of electrical resistivity and photoluminescence confirm high quality material after direct synthesis. If the zone purification would be needed, not many passes through the zone would be necessary, and the time of experiments can be shortened. The next development of our research: the study of the influence of lanthanides on the properties of material the study oft the purification by zone melting after direct synthesis the construction 2 or 3 zones by zone apparatus the study of the mutual relations of the purity of material and photoelectrical and

electrical propertiesgrowing of bulk crystals by Bridgman-Stockbarger method

Fig. 8.2 Electrical characteristics of current versus voltage. Evaluation of resistance

4900 5000 5100 5200 5300

0

5

10

15

PL

inte

ns i

ty[a

rb.u

.]

Wavelength [nm]

BE

F-B(2)

F-B(1)

-LO

-2LO

(a)

(b)

F-B(3)

Fig. 10 Low temerature PL spectra